Chapter Goals (Neurons)After studying this chapter, students should be able to . . .

1. describe the structure of a neuron and explain the functional significance of its principal regions.

2. explain what is meant by the blood-brain barrier and discuss its significance.

3. describe the sheath of Schwann and explain how it functions in the regeneration of cut peripheral nerve fibers.

4. explain how a myelin sheath is formed.

5. define depolarization, repolarization, hyperpolarization, and refractory period.

6. explain the actions of voltage-regulated Na+ and K+ channels and describe the events that occur during the production of an action potential.

7. describe the properties of action potentials and explain the significance of the all-or-none law and the refractory periods.

8. explain how action potentials are regenerated along a myelinated and a nonmyelinated axon.

Chapter Goals9. describe the events that occur in the interval between the electrical excitation of an axon and

the release of neurotransmitter.

10. describe the two general categories of chemically regulated ion channels, and explain how these operate using nicotinic and muscarinic ACh receptors as examples.

11. explain how ACh produces EPSPs, and how IPSPs are produced, and indicate the significance of these processes.

12. compare the characteristics of EPSPs and action potentials.

13. compare the mechanisms that inactivate ACh with those that inactivate monoamine neurotransmitters.

14. explain the role of cyclic AMP in the action of monoamine neurotransmitters, and some of the actions of monoamines in the nervous system.

15. explain the significance of the inhibitory effects of glycine and GABA in the central nervous system.

16. explain how EPSPs and IPSPs can interact and discuss the significance of spatial and temporal summation and of presynaptic and postsynaptic inhibition.

Chapter Goals (CNS)After studying this chapter, students should be able to . . .

1. locate the major brain regions and describe their structures.

2. describe the organization of the cerebrum and the primary roles of its lobes.

3. describe the location and function of the sensory cortex and motor cortex.

4. describe the structures involved in the control of speech and explain their interrelationships.

5. describe the structures included in the limbic system and its role in emotion.

6. describe the location of the thalamus and explain the significance of this organ.

7. describe the location of the hypothalamus and explain the significance of this organ.

8. explain the role of the medulla oblongata in the control of visceral functions.

9. explain how the spinal cord is organized

10. explain the structures and pathways involved in a reflex arc.

Action Potentials

• Action Potential (Transient reversal of polarity) - concept of voltage-activated gates or pores.– a. Reverse polarity via increased permeability

to Na+

– b. Re-establish polarity via permeability to Na+ & permeability to K+

– c. Re-establish original ion concentration via Na+ pump.

Electrical Activity in Axons

• Ion Gates

• Action Potentials

• Conduction of Nerve Impulses– In Unmyelinated Neurons– In myelinated Neurons

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Neurons

• Structure

• Types

• Synaptic Transmission

• Neural Circuits

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Neuron Types

• Myelinated - has myelin sheath. – Speeds impulse conduction via saltatory

conduction (impulse hops from node to node; therefore, more efficient).

– Enhances fidelity of neuron fiber regeneration.

• Non-myelinated - has no myelin sheath

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Synaptic Transmission

• Types of Synapses– Electrical Synapses: Gap Junctions– Chemical Synapses

• Using Acetylcholine

• Using Other Neurotransmitters

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Synaptic Integration

• Long-term Potentiation (Facilitation)

• Inhibition

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Central Nervous System• Organization of NS

– CNS• Brain

• Spinal Cord

– Peripheral NS• Afferent Division (Sensory and Visceral Stimuli)

• Efferent Division– Somatic

– Autonomic

» Sympathetic

» Parasympathetic

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Major Brain Regions• Cerebral Cortex

• Basal Nuclei

• Thalamus

• Hypothalamus

• Cerebellum

• Brain Stem

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Cerebral Cortex• Sensory perception

• Voluntary movement

• Language

• Personality traits

• Knowledge, memory, etc.

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Basal Nuclei• Inhibition of muscle tone

• Coordination of slow, sustained movements

• Suppression of useless movements

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Thalamus• Relay station of synaptic input

• Crude awareness of sensations

• Some degree of consciousness

• Role in motor control

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Hypothalamus• Regulator of ANS

• Link between nervous and endocrine systems

• Involved with emotion and basic behavior

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Cerebellum• Maintenance of balance

• Enhancement of motor tone

• Coordination and planning of skilled voluntary muscle activity

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Brain Stem• Origin of most cranial nerves

• Control of basic life functions

• Equilibrium control

• Reception and integration of all synaptic input from spinal cord; arousal and activation of cerebral cortex

• Sleep centers

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Limbic System (Emotion)

Spinal Cord

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Chapter SummaryNeurons and Supporting Cells

I. The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

A. The central nervous system includes the brain and spinal cord, which contain nuclei and tracts.

B. The peripheral nervous system consists of nerves and ganglia.

II. A neuron consists of dendrites, a cell body, and an axon.

A. The cell body contains the nucleus, Nissl bodies, neurofibrils and other organelles.

B. Dendrites receive stimuli, and the axon conducts nerve impulses away from the cell body.

Chapter SummaryIII. A nerve is a collection of axons in the PNS.

A. A sensory, or afferent, neuron is pseudounipolar and conducts impulses from sensory receptors into the CNS.

B. A motor, or efferent, neuron is multipolar and conducts impulses from the CNS to effector organs.

C. Interneurons, or association neurons, are located entirely within the CNS.

D. Somatic motor nerves innervate skeletal muscle; autonomic nerves innervate smooth muscle, cardiac muscle, and glands.

IV. Supporting cells include Schwann cells and satellite cells in the PNS; in the CNS they include the various types of glial cells; oligodendrocytes, microglia, astrocytes, and ependymal cells.

A. Schwann cells form a sheath of Schwann around axons of the PNS.

B. Some neurons are surrounded by successive wrappings of supporting cell membranes called a myelin sheath. This sheath is formed by Schwann cells in the PNS and oligodendrocytes in the CNS.

C. Astrocytes in the CNS may contribute to the blood-brain barrier

Chapter SummaryThe Synapse

I. Gap junctions are electrical synapses, found in cardiac muscle, smooth muscle, and some regions of the brain.

II. In chemical synapses, neurotransmitters are packaged in synaptic vesicles and released by exocytosis into the synaptic cleft.

A. The neurotransmitter can be called the ligand of the receptor.

B. Binding of the neurotransmitter to the receptor causes the opening of

chemically regulated gates of ion channels.

Chapter SummaryAcetylcholine as a Neurotransmitter

I. There are two different subtypes of ACh receptors: nicotinic and muscarinic.

A. Nicotinic receptors enclose membrane channels and open when ACh bonds to the receptor. This causes a depolarization called an excitatory postsynaptic potential (EPSP) in skeletal muscle cells.

B. The binding of ACh to muscarinic receptors opens ion channels indirectly, through the action of G-proteins. This can cause a hyperpolarization called an inhibitory postsynaptic potential (IPSP).

C. After ACh acts at the synapse it is inactivated by the enzyme acetylcholinesterase (AChE).

II. EPSPs are graded and capable of summation. They decrease in amplitude with distance as they are conducted.

III. ACh is used in the PNS as the neurotransmitter of somatic motor neurons, which stimulate skeletal muscles to contract, and by some autonomic neurons.

Chapter SummaryIV. ACh in the CNS produces EPSPs at synapses in the dendrites or cell body.

These EPSPs travel to the axon hillock, stimulate opening of voltage-regulated gates, and generate action potentials in the axon.

Chapter SummaryMonoamines as Neurotransmitters

I. Monoamines include serotonin, dopamine, norepinephrine, and epinephrine. The last three are also included in the subcategory known as catecholamines.

A. These neurotransmitters are inactivated after being released, primarily by reuptake into the presynaptic nerve endings.

B. Catecholamines may activate adenylate cyclase in the postsynaptic cell, which catalyzes the formation of cyclic AMP.

II. Dopaminergic neurons (those that use dopamine as a neurotransmitter) are implicated in the development of Parkinson’s disease and schizophrenia. Norepinephrine is used as a neurotransmitter by sympathetic neurons in the PNS and by some neurons in the CNS.

Chapter SummaryOther Neurotransmitters

I. The amino acids glutamate and aspartate are excitatory in the CNS.

A. The subclass of glutamate receptor designated as NMDA receptors are implicated in learning and memory.

B. The amino acids glycine and GABA are inhibitory. They produce hyperpolarizations, causing IPSPs, by opening Cl- channels.

II. There are a large number of polypeptides that function as neurotransmitters, including the endogenous opioids.

III. Nitric oxide functions as both a local tissue regulator and a neurotransmitter in the PNS and CNS. It promotes smooth muscle relaxation and is implicated in memory.

Chapter SummarySynaptic Integration

I. Spatial and temporal summation of EPSPs allows a sufficient depolarization to be produced to cause the stimulation of action potentials in the postsynaptic neuron.

A. IPSPs and EPSPs from different synaptic inputs can summate.

B. The production of IPSPs is called postsynaptic inhibition.

II. Long-term potentiation is a process that improves synaptic transmission as a result of the use of the synaptic pathway. This process thus may be a mechanism for learning.

Chapter SummaryStructural Organization of the Brain

During embryonic development, five regions of the brain are formed: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

A. The telencephalon and diencephalon constitute the forebrain; the mesencephalon is the midbrain, and the hindbrain is composed of the metencephalon and the myelencephalon

B. The CNS begins as a hollow tube, and thus the brain and spinal cord are hollow. the cavities of the brain are known as ventricles.

Chapter SummaryCerebrum

I. The cerebrum consists of two hemispheres connected by a large fiber tract called the corpus callosum.

A. The outer part of the cerebrum, the cerebral cortex, consists of gray matter.

B. Under the gray matter is white matter, but nuclei of gray matter, known as the basal nuclei, lie deep within the white matter of the cerebrum.

C. Synaptic potentials within the cerebral cortex produce the electrical activity seen in an electroencephalogram (EEG).

II. The two cerebral hemispheres exhibit some degree of specialization of function, a phenomenon called cerebral lateralization.

A. In most people, the left hemisphere is dominant in language and analytical ability, whereas the right hemisphere is more important in pattern recognition, musical creation, singing, and the recognition of faces.

B. The two hemispheres cooperate in their functions; this is aided by communication between the two via the corpus callosum.

Chapter SummaryIII. Particular regions of the left cerebral cortex appear to be important in

language ability; when these areas are damaged, characteristic types of aphasias result.

A. Wernicke’s area is involved in speech comprehension, whereas Broca’s area is required for the mechanical performance of speech.

B. Wernicke’s area is believed to control Broca’s area by means of the arcuate fasciculus.

C. The angular gyrus is believed to integrate different sources of sensory information and project to Wernicke’s area.

IV. The limbic system and hypothalamus are regions of the brain that have been implicated as centers for various emotions.

V. Memory can be divided into short-term and long-term categories.

A. The medial temporal lobes, in particular the hippocampus and perhaps the amygdaloid nucleus, appear to be required for the consolidation of short-term memory into long-term memory.

B. Particular aspects of a memory may be stored in numerous brain regions.

C. Long-term potentiation is a phenomenon that may be involved in some aspects of memory.

Chapter SummaryDiencephalon

I. The diencephalon is the region of the forebrain which includes the thalamus, epithalamus, hypothalamus, and pituitary gland.

A. The thalamus serves an important relay center for sensory information, among its other functions.

B. The epithalamus contains a choroid plexus, where cerebrospinal fluid is formed. The pineal gland, which secretes the hormone melatonin, is also part of the epithalamus.

C. The hypothalamus forms the floor of the third ventricle, and the pituitary gland is located immediately inferior to the hypothalamus.

II. The hypothalamus is the main control center for of visceral activities.

A. The hypothalamus contains centers for the control of thirst, hunger, body temperature, and (together with the limbic system) various emotions.

B. The hypothalamus regulates the secretions of the pituitary gland. It controls the posterior pituitary by means of a fiber tract, and it controls the anterior pituitary by means of hormones.

Chapter SummaryMidbrain and Hindbrain

I. The midbrain contains the superior and inferior colliculi, which are involved in visual and auditory reflexes, respectively and nuclei that contain dopaminergic neurons that project to the corpus striatum and limbic system of the forebrain.

II. The hindbrain consists of two regions: the metencephalon and the myelencephalon.

A. The metencephalon contains the pons and cerebellum. The pons contains nuclei for four pairs of cranial nerves, and the cerebellum plays an important role in the control of skeletal movements.

B. The myelencephalon consists of only one region, the medulla oblongata. The medulla contains centers for the regulation of such vital functions as breathing and the control of the cardiovascular system.

Chapter SummarySpinal Cord Tracts

I. Ascending tracts carry sensory information from sensory organs up to the spinal cord to the brain.

II. Descending tracts are motor tracts and are divided into two groups: the pyramidal and the extrapyramidal systems.

A. Pyramidal tracts are the corticospinal tracts. They begin in the precentral gyrus and descend, without synapsing, into the spinal cord.

B. Most of the corticospinal fibers decussate in the pyramids of the medulla oblongata.

C. Regions of the cerebral cortex, the basal nuclei, and the cerebellum, control movements indirectly by synapsing with other regions that give rise to descending extrapyramidal fiber tracts.

D. The major extrapyramidal motor tract is the reticulospinal tract, which has its origin in the reticular formation of the midbrain.

Chapter SummaryCranial and Spinal Nerves

I. There are twelve pairs of cranial nerves. Most of these are mixed, but some are exclusively sensory in function.

II. There are thirty-one pairs of spinal nerves. Each of these contains both sensory and motor fibers.

A. The dorsal root of a spinal nerve contains sensory fibers, and the cell bodies of these neurons are contained in the dorsal root ganglion.

B. The ventral root of a spinal nerve contains motor fibers.

III. A reflex arc is the pathway that involves a sensory neuron and a motor neuron; one or more association neurons also may be involved in some reflexes.